In U.S. Pat. No. 5,476,572 and in co-pending U.S. application Ser. No. 08/354,005, filed Dec. 5, 1994 (the disclosures of which are incorporated by reference herein), novel methods and systems are disclosed for feeding comminuted cellulose fibrous material to a continuous digester by means of a chip slurry pump. These inventions vastly reduce the size and cost of conventional feeding systems. Although these systems can effectively feed a slurry of chips and liquor to a digester, they may not be able to withstand changes in slurry liquor volume in the chute preceding the pump.
In the system of the co-pending applications, the chip and liquor slurry passes from a metering device through a conduit, i.e., a chute, directly to the slurry pump. However, small changes in the liquid volume of the slurry can result in gross changes in the height of the liquid level in the relatively narrow chute. The chute diameter/width is preferably narrow to ensure a high slurry velocity in the chute. A high velocity minimizes the potential for chips floating in the chute and ensures a more uniform feed to the pump. The present invention minimizes the effect of changes in liquid volume upon the operation of such a system while providing an advantageous system and method for preferentially varying the liquid level in the feed system. The present invention also permits the separate control of liquor volume and chip volume in a feed system in which chips are pumped.
One aspect of the invention relates to a system for feeding comminuted cellulose material entrained in liquid to a high pressure feeder connected to a digester (preferably a continuous digester). The system comprises the following components: A vertical pretreatment (e.g. steaming) vessel having a discharge at the bottom thereof. A metering device connected to the discharge of the steaming vessel. A generally vertical chute extending downwardly from the metering device. A slurry pump for pumping a slurry of comminuted cellulosic material in liquid, the slurry pump having an inlet, and the pump connected to a high pressure feeder. The chute operatively connected to the slurry pump inlet. A substantially vertical liquor tank having a top and a bottom, the bottom including a discharge opening therein. And, a conduit directly connecting the liquor tank discharge opening with the slurry pump inlet. Preferably the liquor tank is in direct communication with the chute, and through the chute the metering device in the steaming vessel, through the discharge opening in the liquor tank bottom. Means are also preferably provided for maintaining a predetermined liquid level in the liquor tank, which in turn maintains a liquid level in the metering device and steaming vessel.
The metering devices may comprise a wide variety of structures, such as a conventional chip meter (having a star feeder configuration), a submerged chip meter, a metering screw, or the like. The system may be maintained substantially at atmospheric pressure, or it may a superatmospheric pressure system, in which case an isolating device (such as a low pressure feeder) is provided between the steaming vessel and the slurry pump for isolating the slurry pump from the atmosphere and for maintaining the slurry pump at superatmospheric pressure. For example a low pressure feeder may be provided between the steaming vessel and the metering device, or at other locations such as between the metering device and the feed chute, or even between the feed chute and the inlet to the slurry pump.
The system is also preferably provided in combination with a high pressure feeder having a low pressure inlet, low pressure outlet, high pressure inlet connected to a high pressure pump, and high pressure outlet. The slurry pump is connected to the low pressure inlet of the high pressure feeder, and the high pressure outlet is connected to the digester, e.g. the top of a continuous digester.
The system may also further comprise an in-line drainer, a level tank, a valve that is always at least partially open, a first recirculation conduit connecting the low pressure outlet to the drainer, a second conduit connecting the drainer to the level tank so that liquid drained from the drainer passes into the level tank; and a third recirculation conduit connecting the drainer to the discharge opening at the bottom of the liquor tank.
The steaming vessel may be of a wide variety of configurations. Preferably, however, either it or an outlet from the metering device or the feed chute, includes a single convergent outlet with side relief, such as shown in co-pending application Ser. No. 08/189,546 filed Feb. 1, 1994 now U.S. Pat. No. 5,500,083 and Ser. No. 08/366,581 filed Dec. 30, 1994, the disclosures of which are incorporated by reference herein. While the preferred pretreatment vessel is a steaming vessel, other vessels can be utilized to pretreat the chips or other cellulose material with a cooking liquor (such as kraft white liquor, black liquor, green liquor, sulfite liquor, or soda liquor), or to pretreat it with anthraquinone (or its derivatives or equivalents), or sulfide enhancing additives (such as polysulifide or NaSH, or hydrosulfide gas) or simply with hot water.
According to another aspect of the present invention a method of feeding a slurry of comminuted cellulosic fibrous material (such as wood chips) in liquid (such as cooking liquor like white liquor) to a digester is provided, which utilizes a pretreatment vessel, high pressure feeder having a low pressure inlet, and slurry pump having an inlet. The method comprises the steps of: (a) Pretreating the comminuted cellulosic fibrous material in the pretreatment vessel. (b) Metering the flow of comminuted cellulosic fibrous material from the pretreatment vessel. (c) Entraining the metered comminuted cellulosic fibrous material in liquid to form a slurry. (d) Feeding the slurry to the inlet to the slurry pump. (e) Providing a separate supply of liquid to the slurry pump to facilitate the transfer of comminuted cellulosic fibrous material to the pump. (f) Transporting the slurry under the influence of the slurry pump to the low pressure inlet to the high pressure feeder. And, (g) feeding the comminuted cellulosic fibrous material with the high pressure feeder to a digester. The preferred pretreatment is steaming, but other treatments--as described above with respect to the pretreatment vessel (e.g. with polysulfide)--may be provided instead of or in addition to presteaming.
Step (e) is preferably practiced by connecting an open bottom discharge from a substantially vertical liquid tank that is higher than the slurry pump inlet directly to the slurry pump inlet. There is also preferably the further step of maintaining a liquid level in the substantially vertical liquid tank, and providing an open connection between the vertical liquid tank open bottom discharge and the steaming vessel so as to provide the same liquid level in the steaming vessel. Steps (c) and (d) are typically practiced to entrain the cellulose material in cooking liquor.
According to yet another aspect of the present invention a slurry feeding system for feeding a slurry of comminuted cellulosic fibrous material in liquid to a digester is provided. The system comprises the following components: A substantially vertical pretreatment vessel. Means for metering the flow of comminuted cellulosic fibrous material from the pretreatment vessel. Means for entraining the metered comminuted cellulosic fibrous material in liquid to form a slurry. A slurry pump having an inlet. Means for feeding the slurry to the inlet to the slurry pump. Means providing a separate supply of liquid to the inlet to the slurry pump to facilitate the transfer of comminuted cellulosic fibrous material to the slurry pump to permit separate control of liquor and comminuted cellulosic material volumes, and to minimize the effect of changes in liquid volume upon operation. A high pressure feeder having a low pressure inlet and a high pressure outlet. Means for transporting the slurry under the influence of the slurry pump to the low pressure inlet of the high pressure feeder. And, means for transporting the comminuted cellulosic fibrous material from the high pressure feeder to a digester. The means for providing a separate supply of liquid preferably comprises a substantially vertical liquid tank having an open bottom discharge that is higher than the slurry pump inlet, and connected directly to the slurry pump inlet. This invention also preferably includes a means for thermally separating liquors between the high-pressure transfer device and the digester.
It is the primary object of the present invention to provide a system and method which accommodate variations in liquor volume for the advantageous feeding of comminuted cellulosic fibrous material slurry to a digester, retaining all of the other advantages of the structures of co-pending application Ser. No. 08/354,005 and U.S. Pat. No. 5,476,572, incorporated by reference herein, including utilization of the high pressure feeder at ground level. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.